With the mounting of semiconductor chips on a substrate having a solder portion a semiconductor chip held by a gripper spring mounted on a bondhead is lowered onto the substrate. In doing so, the gripper is deflected towards the bondhead. Subsequently, the semiconductor chip is raised by a predetermined distance and then released. Optionally, the semiconductor chip is moved up and down before being released. With this method, the semiconductor chip is under mechanical control until the solder has taken up a stable form and the semiconductor chip has achieved its final position.
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7. Apparatus for mounting semiconductor chips onto a substrate having a solder portion with a bondhead with a gripper for grasping a semiconductor chip, whereby the gripper is deflectable towards the bondhead and wherein a sensor is present which indicates whether the gripper is deflected.
1. Method for mounting a semiconductor chip on a substrate having a solder portion, the method comprising the following steps:
a) Presenting the substrate on a support; b) Grasping the semiconductor chip by means of a gripper spring mounted on a bondhead; c) Lowering the semiconductor chip onto the substrate, whereby the gripper carrying the semiconductor chip is deflected towards the bondhead; d) Raising the semiconductor chip by a predetermined distance; e) Releasing the semiconductor chip, and f) Moving the bondhead away.
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The invention concerns a method for mounting semiconductor chips and an apparatus suitable for carrying out the method.
Soldering methods of this kind are typically--however not exclusively--used for the mounting of semiconductor chips on a metallic carrier, a so-called leadframe. Above all, power semiconductors are bonded to the substrate (normally copper) as a rule with soft solder in order to guarantee effective dissipation of the heat loss created in operation by the semiconductor chip by means of the solder connection.
An apparatus for the application of liquid solder to a substrate and for the subsequent mounting of a semiconductor chip onto the liquid solder portion is known from the U.S. Pat. No. 5,878,939. Under the name of "Die Bonder 2007 SSI" the applicant offers an apparatus of this kind whereby the semiconductor chip is placed over the solder portion on the substrate by means of a process known as "Overtravel". With this process, the bondhead is lowered so far that, on impact with the solder portion, the gripper holding the semiconductor chip is deflected towards the bondhead. In this way, variations in the thickness of the semiconductor chip, which typically amount to up to 40 μm, and variations in the height of the surface of the substrate can be overcome without problem.
The liquid soft solder has a very low viscosity and, on impact of the semiconductor chip, behaves like water, ie, it flows easily and almost instantly from underneath the semiconductor chip and spreads outside the semiconductor chip. Examinations with a high-speed camera have shown that in doing so a gap remains between the semiconductor chip and the substrate which is in fact filled with solder but the thickness of which only amounts to several micrometres. When the bondhead moves away, ideally a large part of the solder flows back into the gap between the semiconductor chip and the substrate, whereby the gap is again increased, the thickness of the solder layer however is subject to certain deviations even when the semiconductor chip is moved back and forth parallel to the surface of the substrate. Often, the solder does not flow back under the semiconductor chip but settles in beads next to the semiconductor chip. This results in very thin solder layers. In event that the solder flows back only partially or on one side, this results in solder layers with large tilt. The flowing back of the solder and the formation of a solder layer of sufficient thickness and homogeneity takes place uncontrolled. Increasingly higher demands are however now placed on the soldered connection: uniform thickness of the solder layer, distribution of the solder layer over the entire surface of the chip, completely bubblefree, high purity of the soldered connection.
The object of the invention is to further improve the quality of the soldered connection between the semiconductor chip and the substrate.
The method in accordance with the invention for the mounting of semiconductor chips on a substrate having a solder portion is characterized by means of the following steps:
a) Presenting the substrate on a support;
b) Grasping the semiconductor chip by means of a gripper spring mounted on a bondhead;
c) Lowering of the semiconductor chip onto the substrate whereby the gripper carrying the semiconductor chip is deflected towards the bondhead;
d) Lifting the semiconductor chip by a predetermined distance;
e) Releasing the semiconductor chip;
f) Moving the bondhead away.
With step c, first of all the semiconductor chip impacts on the liquid solder whereby the solder portion is at first pressed flat as the result of the impact and then a large part of the solder is pressed out of the gap between the semiconductor chip and the substrate. The solder collects in beads to the side of the semiconductor chip. A gap filled with solder remains between the semiconductor chip and the substrate the thickness of which amounts to only a few micrometres, typically only around 5 micrometres. The aim is that with this step c, the entire back of the semiconductor chip including the comers is wetted with solder and that the solder gathers as uniformly as possible on all four sides of the semiconductor chip. How and whether the wetting of the entire back of the semiconductor chip takes place is not only dependent on the impact speed of the semiconductor chip and the degree of parallelism of its back with the surface of the substrate but also on other factors such as for example the cleanliness of the back of the semiconductor chip and the area on the substrate to be wetted, the form and quality of the solder, etc.
With step d, the semiconductor chip is now, unlike prior art, brought to a predetermined height above the substrate under mechanical control of the gripper whereby solder flows back into the increasing gap between the semiconductor chip and the substrate. The flowing back of the solder takes place under controlled conditions, in particular under the controlled suction effect of the semiconductor chip guided by the gripper. In this way it is achieved that the gap is relatively homogenously filled with solder up to the comers of the semiconductor chip. The wetting is meanwhile concluded, ie, the solder has formed meniscuses and has therefore achieved a stable state. The mechanical connection between the semiconductor chip and the gripper can now be released and the bondhead can be moved away without the position and/or tilt of the semiconductor chip being altered.
It is possible that bubbles, so-called voids, are contained within the solder. By means of a controlled up and down movement of the semiconductor chip after step d, it can if necessary be achieved that possible large bubbles break up into several small bubbles and that the bubbles migrate towards the edge of the semiconductor chip whereby bubbles arriving at the edge disappear. The reduction or even complete elimination of the bubbles, which itself is a characteristic of quality, also has the effect that the position of the semiconductor chip alters less strongly or not at all when releasing from the gripper, ie, improved tilt and therefore a higher value of the thinnest part of the solder layer or a more uniform thickness of the solder layer. As long as larger bubbles exist, there is the danger that the semiconductor chip sinks locally and ends up in a slanted position in relation to the surface of the substrate.
A prerequisite for achieving the required quality of the solder layer is of course that the bondhead is adjusted so that the underneath of the semiconductor chip is parallel to the surface of the substrate.
In the following, embodiments of the invention are explained in more detail based on the drawing.
is It is shown in:
After the bondhead 4 has reached the height H0 and the solder 2 has collected to the side of the semiconductor chip 1, the clamp jaws 8 are moved together so that they clamp the gripper 5 securely to the bondhead 4 in the deflected condition. With step d the bondhead 4 is raised by a predetermined distance H1 with which the clamped gripper 5 and the semiconductor chip 1 are also raised by the distance H1.
It has now been proved, that the tilt of the placed semiconductor chip 1 can be significantly improved when, after step d, the gripper 5 is additionally moved once or several times towards and away from the substrate 3 in z direction, ie, when the width of the gap between the semiconductor chip 1 and the substrate 3 is varied in a controlled manner by means of upward and downward movements of the bondhead 4 and the gripper 5. With this "pumping" it is achieved that, on the one hand, the solder 2 flows along the edges of the semiconductor chip 1 and a complete soldered seam 2a forms around the semiconductor chip 1 (see FIG. 2). On the other hand, any bubbles which may exist are diminished and transported to the edge of the semiconductor chip 1. Altogether this results in a complete, ie, bubble-free and uniform distribution of the solder 2 in the gap which manifests itself in less tilt and therefore an associated higher value of the thinnest part of the solder 2.
The upward and downward movement of the bondhead 4 can take place with the drive which drives the bondhead 4 in vertical direction. However, the upward and downward movement of the bondhead 4 is preferably brought about by means of an eccentric disc which acts directly on the bondhead 4. In order that the movements of the eccentric disc are perfectly transmitted to the bondhead 4, it is necessary that the bondhead 4 and the eccentric disc are continuously in contact. This is achieved in that the bondhead 4 is designed so that it comes into contact with the eccentric disc on moving to the bond position and ultimately presses against it with a slight initial tension in the bond position. In this way, oscillations of the bondhead 4 in z direction are at the same time effectively prevented.
With this embodiment, steps c and d therefore take place as follows:
c) Lowering of the bondhead 4 to a predetermined height above the support 9, whereby the height is predetermined so that the gripper 5 holding the semiconductor chip 1 is deflected towards the bondhead 4, and
d) Securing the gripper 5 to the bondhead 4 and raising the bondhead 4 by the predetermined distance H1;
The chronological sequence of steps e and f can be planned on the one hand so that after releasing the vacuum at point in time t4 a predetermined period of time T45 is waited and the bondhead 4 is only moved away afterwards. The period of time T45 must be measured so that the bondhead 4 loosens itself from the semiconductor chip 1 without problem when moving away. On the other hand, a sensor can be foreseen which measures the strength of the vacuum and the moving away of the bondhead 4 takes place at point in time t5 as soon as the vacuum falls below a predetermined value.
An advantageous solution which enables the fast reduction of the vacuum at the tip of the gripper 5 comprises the provision of a drill hole 10 (
It has been proved that it can be advantageous to reduce the impact speed of the semiconductor chip 1 on the solder 2 so that the displaced solder 2 collects as close as possible to the semiconductor chip 1 and instead to move the semiconductor chip 1 at least once, preferably several times, upwards and downwards. The up and down movement of the semiconductor chip 1 namely, promotes the complete wetting and coverage of the back of the semiconductor chip 1 with solder 2 or the formation of the soldered seam 2a running completely around the semiconductor chip 1.
Instead of the clamp jaws 8, other means can be foreseen in order to lock the gripper 5 onto the bondhead 4. For example, the gripper 5 can be pressed against the bondhead 4 by means of a lever rotatable on an axis and actuated by an electromagnet or can be fixed onto the bondhead 4 by means of vacuum.
Instead of locking the gripper 5 onto the bondhead 4 and raising the bondhead 4 by the distance H1, a solution can be foreseen with which the gripper 5 is raised by the distance H1 in relation to the bondhead 4.
In addition, with the sensor 11, it can be checked whether the gripper 5 jams on the bondhead 4. This is the case when both contacts of the sensor 11 remain either always open or always closed.
As described for the first embodiment, an up and down movement of the bondhead 4 with the gripper 5 can also take place here after step d and here also with step e, after releasing the vacuum, either firstly a predetermined period of time can be waited and then the bondhead 4 moved away or the moving away of the bondhead at step f can only take place when the vacuum holding the semiconductor chip 1 falls below a predetermined value. In doing so, the closing of the contacts of the sensor 11 can also be used to trigger the point in time at which the vacuum is to be released.
With certain solder 2--substrate 3 material pairs, it is possible to move the bondhead 4 directly away without the up and down movement. It is then advantageous to already release the vacuum before reaching the height H0+H4 so that the bondhead 4 can move away without dwelling at height H0+H4. It is additionally advantageous to control the strength of the vacuum and to provide the gripper 5 with the drill hole 10 serving as a controlled leak so that the period of time c, which stretches from switching off the vacuum up to releasing the semiconductor chip 1 from the gripper 5, is reproducible. Then the vacuum can be released around this period of time X before reaching the height H0+H4 corresponding to the trajectory of the bondhead 4. With this movement also, the semiconductor chip 1 is mechanically guided until reaching the predetermined height H0+H4.
With other solder 2--substrate 3 material pairs however, it is better to move the bondhead 4 up and down after reaching the height H0+H4.
The method according to the invention results in excellent solder connections when the semiconductor chip is mounted on a pressed flat solder portion (see EP 852983) the linear dimensions of which are somewhat larger than the linear dimensions of the semiconductor chip. The method in accordance with the invention also results in very good soldered connections when the semiconductor chip is mounted on a drop-shaped solder portion. The various parameters of the method in accordance with the invention enable an optimum adaptation of the movement sequence of the semiconductor chip to the conditions dictated by the solder--substrate material combination. The mechanically controlled movement of the semiconductor chip carried out up to achieving its final position with a stable formed solder layer enables lower process temperatures, i.e., the substrate and the solder must only be heated up to a few °C C. above the melting temperature of the solder. With the previously known methods, process temperatures which lie 50 or 80°C C. above the melting temperature of the solder are necessary so that the solder flows back.
Luechinger, Christoph, Limacher, Markus
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Jul 03 2000 | LUECHINGER, CHRISTOPH | ESEC Trading SA | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 011096 | /0341 | |
Jul 12 2000 | LIMACHER, MARKUS | ESEC Trading SA | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 011096 | /0341 | |
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